tools/fakeroot: explicitly pass CPP variable

For some reason, the generated configure script fails to properly set up
the internal preprocessor command variable, causing the host OS check for
Darwin to fail after the last update.
Explicitly setting CPP fixes this issue

Signed-off-by: Felix Fietkau <nbd@nbd.name>

tools/fakeroot/Makefile

@@ -19,7 +19,8 @@ include $(INCLUDE_DIR)/host-build.mk
 
 HOST_CONFIGURE_VARS += \
 	ac_cv_header_sys_capability_h=no \
-	ac_cv_func_capset=no
+	ac_cv_func_capset=no \
+	CPP="$(HOSTCC) -E"
 
 HOST_CONFIGURE_ARGS += \
 	--with-ipc=tcp